HDAC4 belongs to class IIa family of histone deacetylases which were traditionally named for their ability to deacetylate lysine residues on nuclear histone proteins and to repress gene expression epigenetically. However, in the last few decades these HDACs have been found to regulate many non-histone proteins both in the nucleus as well as in the cytoplasm (reviewed by Yao and Yang in J Biomed Biotechnol. (2011) 2011:146493). Characteristic features of class IIa HDACs include (i) presence of a conserved N-terminal regulatory domain, containing NLS (nuclear localization signal) for nucleocytoplasmic shuttling, and binding motifs for transcription factors and corepressors; (ii) tissue specific expression and; (iii) responsiveness to phosphorylation mediated external/internal stimuli (Parra and Verdin, Curr Opin Pharmacol. (2010) 10(4):454-60).
High HDAC4 expression is seen in cardiac and smooth muscles, heart and brain. HDAC4 can inhibit the expression of many genes by binding with tissue specific transcription factors (e.g. MEF2, Runx2, p53 and SRF) in association with corepressors (e.g. N—CoR and SMRT), and other HDACs (HDAC3 and 5) (Parra and Verdin, 2010, ibid.). A number of studies relate the abnormal HDAC4 expression and subcellular localization to developmental defects and neurodegenerative diseases (Majdzadeh et al., Front Biosci. (2008) 13:1072-82). In response to a specific cell stimulus, a variety of kinases (mainly CAMKs) can phosphorylate HDAC4 at conserved serine residues (Ser-246, Ser-467 and Ser-632 in humans), creating a docking site for 14-3-3 protein, which entraps HDACs in the cytoplasm, thus reliving the target promoters from HDAC mediated repression. PP2A and PP1 phosphatases mediated dephosphorylation, on the other hand, has been shown to expose the HDAC4 NLS and promote its nuclear import (Parra and Verdin, 2010, ibid).
Wilson et al. reported in Mol Biol Cell. (2008) 19(10):4062-75, a strong HDAC4 expression in the proliferating mouse colon crypts. Silencing of HDAC4 or a few other HDACs, as well as treatment with pan-HDAC inhibitors, have been demonstrated to inhibit the cancer cell proliferation via upregulation of p21 either directly or indirectly via p53 under DNA damaging conditions (Basile et al., J Biol Chem. (2006) 281(4):2347-57; Wilson et al., 2008, ibid). In a high-throughput study, human breast tumor samples showed significant HDAC4 overexpression suggesting potential role of HDAC4 in human cancers (Witt et al., Cancer Lett. (2009) 277(1):8-21).
More recent findings have identified a number of cytoplasmic targets, which highlight the role of HDAC4 in regulation of development, angiogenesis, apoptosis and chemoresistance. Activity of HDAC4 and its association with HIF1α in the cytoplasm was shown to be required for the survival of retinal neurons (Chen and Cepko, Science. (2009) 323(5911):256-9). HDAC4 mediated deacetylation of HIF1α N-terminal lysines stabilizes HIF1α, and promotes transcription of its target genes namely VEGF and glycolytic genes (LDHA and Glut1) (Geng et al., J Biol Chem. (2011) 286(44):38095-102). Thus, HDAC4 appears to prepare cells to adapt to hypoxic/stress conditions and also contribute to tumor angiogenesis. Importantly, in this report, prostate cancer cells silenced for HDAC4 were more responsive to docetaxel treatment under hypoxic conditions.
HDAC4 overexpression is also shown to enhance the cisplatin-resistance in ovarian cancer by activation and nuclear translocation of STAT1 (deacetylation followed by phosphorylation). Instead, more specific HDAC4 inhibitor, APHA4a, induced caspase activity and restored cisplatin-sensitivity (Stronach et al., Cancer Res. (2011) 71(13):4412-22).
Most recently, the importance of both nuclear and cytoplasmic HDAC4 in neuronal survival and ataxia telangiectasia (AT) pathogenesis was shown (Li et al., Nat Med. (2012) 18 (5): 783-790). Enhanced PP2A activity, due to loss of ATM, was shown to promote HDAC4 nuclear accumulation and epigenetic repression of various promoters. Conversely, cytoplasmic HDAC4 inhibited the cell-cycle re-entry and caspase-3 activation. Importantly, these findings are of great importance also in the cancer field, since PP2A activation may prove to be useful for improved killing of cancerous cells in the brain by shifting the pro-survival cytoplasmic HDAC4 to anti-survival nuclear HDAC4.
Given that cancer is a devastating disease affecting all communities worldwide and that either intrinsic or acquired resistance is the major problem related to currently used chemotherapies, there is an identified need in the art for new cancer therapy regimens inducing apoptosis.